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Switching from gas to electric hot water and heating

user-2337692 | Posted in General Questions on

We just recently finished a renovation/extension in Seattle of a 1920s house (2 story + basement) that included installing 10kw of solar panels. The entire roof was replaced with R-50 insulation and all the new walls are R-22 (about 1/3 of the existing structure). Pre-renovation blower door test was 0.35 ACHn – we haven’t done an updated test yet.

For heating, we have a 1.5 ton Mitsubishi ducted heat pump for the upstairs floor (830 sqft). For the basement (500 sqft) and main floor (780 sqft) we have a gas water boiler with a hydronic air handler to a ducted air furnace. On the main floor we also have hydronic in-floor heat for 200 sqft of concrete floor – short term experience is this keeps at least half of the main floor reasonably warm. 

With the panels, we’d like to switch our gas hot water and heating on the main/basement  floor to electric. Here are the options I’m considering:
Option 1. Replace main floor and basement with another 1.5 ton Mitsubishi ducted heat pump. This would have the advantage of cooling but we find having cooling in the bedrooms only to be more than sufficient,. Replace gas water boiler with hybrid heat pump water heater for domestic hot water and in-floor radiant heat

Option 2. Replace gas water boiler with a larger hybrid heat pump water heater with existing connections to hydronic air handler and in-floor radiant. The issue with this option is that the location of the gas boiler is in a conditioned basement space which as I understand would not be logical. It is just a storage/mechanical room so we could convert it to an unconditioned space. It is already partly insulated against the rest of the basement (2 internal walls) but more would insulation work would need to be done (ductwork, basement ceiling). 

My questions:
– Which of these options would be most efficient?
– Option 2 would be cheaper but does converting the mechanical room to unconditioned space make sense?
– Are there other options I should consider?

Thanks in advance!

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  1. m854 | | #1

    Have you looked at the Sanden CO2 split heat pump water heaters? They might work for you to do hot water and at least some of the hydronic load. Small Planet Supply says "For homes with a design temp of 12°F or higher, and a heat load of 8kbtu/hr or less, this can be the entire DHW and space conditioning system."

    I don't think it makes sense to heat the house with a heat pump water heater that's fully located indoors. Even if you make the space unconditioned, it would need a lot of airflow to outside for it to work well, and the ones I've seen don't work below about 37F. You could duct it to outside, and that might be a little better. Also consider that most of the Hybrid heat pump water heaters have a pretty small heat output from the heat pump. For example I think the Rheem ones are only about 4,200 BTU. They count on using storage to meet hot water demand and then slowly recovering, which doesn't help if you have a continuous heating demand. If you know of more powerful ones, let me know.

  2. Expert Member
    BILL WICHERS | | #2

    You can’t heat your house with a normal heat pump hot water heater located indoors. The reason is that heat pumps work by MOVING, not making, heat from one place to another. Heat pump hot water heaters generally operate by scavenging (fancy way to say “taking”) heat from the area around them, usually a basement, and using it to heat the water in the tank. The result is hotter water in the tank, but colder air in the basement.

    In your case, you’d be attempting to move heat from the basement to your radiant floors. That will work for a while, but you’ll end up pumping heat to the floors which will leak into the basement to be pumped back to the floors which will be leaked to the basement... you’re not actually putting any new thermal energy into your house this way except whatever losses are coming from the motors in the heat pump water heater.

    If you can get a heat pump water heater with an outdoor coil that would work, otherwise I’d go with another mini split and keep your gas fired boiler and use the radiant heat as backup heat on really cold days when the heat pumps aren’t as efficient.


  3. Expert Member
    Dana Dorsett | | #3

    >"Replace gas water boiler with hybrid heat pump water heater for domestic hot water and in-floor radiant heat."
    >"Replace gas water boiler with a larger hybrid heat pump water heater with existing connections to hydronic air handler and in-floor radiant."

    That doesn't work with a tank-top system, since it would be pulling the heat from one part of the house to heat another part of the house, and they're probably not big enough to heat very much space. Even if it has enough to cover the space heating load, the recovery time after a shower could be insanely long, even in hybrid mode.

    With a Sanden (which has an outdoor coil) it could work but it's expensive, and would require doing all the math. It's good for about 15,000 BTU/hr @ Seattle type design temperatures, which can probably cover either your 200' of radiant floor or your hot water needs, but not both. You''ll have to do the math.

    A 500' basement is large enough to do the domestic hot water with a tank-top type HPWH. If the foundation walls aren't insulated to the code-minimum R15 continuous insulation, they should be.

    I suspect your 780' first floor + 500' basement load combined are well under the output of a cool-climate 1-ton ducted mini-split, and would have real margin if the basement walls were insulted to code. The Fujitsu 12RLFCD can deliver nearly 18,000 BTU/hr @ +17F.!/product/25311

    If you need a beefier air handler than that the ARU12RGLX works:!/product/25349

    By comparison the Mitsubishi SUZ KAxx/SEZ KDxx series crap out on capacity much sooner, and have even wimpier blower (that still might work with what is likely to be way oversized ducts.)

  4. user-2337692 | | #4

    Thanks for the responses. I'm investigating the Sanden heat pump through a local installer but sounds like the better option for heating is to go with the ducted heat pump for the main/basement floors.

    I saw someone else mention the Gree multifunction air to water heat pumps that are available in Aus/NZ. Anyone know when these might be available in NA?

  5. user-2337692 | | #5

    I should have added that our gas boiler is 25+ years old so it's due for replacement.

  6. Expert Member
    Dana Dorsett | | #6

    Also sold in the US, LG's Multi-Vs heat pump supports a hydronnic output option as well as most of their ducted & ductless heat pump heads, supporting multiple zones. I suspect it takes at least the 3 ton, or maybe even the 5 ton if you want to do both hydronic heat and domestic hot water with it. But in your climate it's efficiency is quite high. With a "reverse indirect" (eg ErgoMax E24 or Turbomax 24 or larger, depending on your hot water needs) as the buffer tank for the hydronic heating system it can work quite well with radiant floors and other low temp radiation.

    One cool thing about the Multi Vs is that it can air condition and heat separate zones (including making hot water) at the same time with ultra-high efficiency.

    Since you have a heating history on the gas boiler, run a fuel-used based load calculation on the zones it's currently heating, which will give us a much better idea as to how much load the heat + HW mechanical systems need to cover.

    Last year This Old House did a major deep energy retrofit and house expansion on a house in Rhode Island that uses a 5 ton Multi Vs for the whole shebang (and a mini-split for the small detached shop building.) The goal was Net Zero Energy with PV mounted on the shop building. The 99% outside design temperature in that location is about 10F cooler than Seattle's and it was (after the expansion) a bigger house than yours.

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